In a holographic data storage (HDS) device or system, data-bearing holograms are recorded in a photosensitive holographic storage medium. The data in a hologram may be recovered by illuminating the holographic storage medium with a probe beam (also referred to as a recovery or reconstruction beam), which is nominally a replica of the reference beam used to record (write) the hologram. The probe beam is diffracted by the recorded hologram, thereby generating a diffracted beam, which is a replica of the original data-bearing signal beam. The diffracted beam may then be detected with a detector array, such as a camera, and the data recovered from the resulting detected holographic image.
The quality of the diffracted beam often depends on the precise alignment of the probe beam wavefront to the holographic fringes within the hologram. High storage capacity may be achieved by densely multiplexing many holograms within a medium. An HDS device or system often employs one or more multiplexing methods, each of which may require one or more control signals to select the desired hologram to be recovered. The HDS device or system may, for example, use a galvanometer in order to control the probe beam angle of incidence to effect angle multiplexing, and translation plus rotation (r, θ) actuators in order to properly position the disk-shaped medium with respect to the probe beam and recovery optics (spatial multiplexing). See Mok, “Angle-Multiplexed Storage of 5000 Holograms in Lithium Niobate,” Opt. Lett. 18:915-917 (1993). Other control signals may be used to adjust other alignment parameters not associated with multiplexing per se.
A variety of methods have been used to try to align a probe beam with respect to the hologram(s) to be recovered from the holographic storage medium. The simplest method, which may be adequate in some cases, is to simply apply an open-loop control signal. In other cases, an actuator which may contain an integrated feedback sensor, such as a linear or angular position encoder, may be used to try to position the probe beam and/or holographic storage medium properly with respect to the hologram(s) to be recovered. Servo marks may also be pre-formatted onto the holographic storage medium in order to provide positioning feedback. See U.S. Pat. No. 7,184,383 (Ayres et. al.), issed Feb. 27, 2007; Horimai et al., “Collinear Holography,” Appl. Opt. 44:2575-2579 (2005). Methods involving wavelength and temperature tuning have also been used. See U.S. Pat. Appln. No. 2007/0211321(Hoskins et al.), published Sep. 13, 2007.